1. Field of Invention
This invention relates to a switching power supply which produces a DC voltage of a preset value at a circuit on the secondary side of a transformer and feeds an output signal thereof back to the primary side of the transformer while electrically isolating the output signal.
2. Description of the Prior Art
FIG. 1 depicts a conventional switching power supply, wherein a DC converting circuit 10 receives a DC voltage Vin from a DC source 1 and converts the DC voltage into a stable output voltage V.sub.o. A transformer 3 is provided to isolate the output side from the input side. For example, DC source 1 is a circuit for rectifying and smoothing AC power from an AC power source. Depending on the condition of the power source, this voltage may vary greatly, or a surge voltage or current may be superimposed on the voltage. If such an overvoltage or overcurrent is applied directly to a load 25, the load 25 may be damaged. Thus, transformer 3 provides isolation between the circuit connected to the primary side of transformer 3 and the circuit connected to the secondary side thereof.
DC converting circuit 10 has DC source 1 connected to one end of primary winding 5 of transformer 3, while a switching element or device Q1 is connected to the other end of primary winding 5. A rectifying-smoothing circuit 9 is connected to secondary winding 7 of transformer 3. Circuit 9 comprises a rectifying diode D1 and a low-pass filter including a smoothing choke coil L1 and a smoothing capacitor C1. When diode D1 is cut off, a flywheel diode D2 continuously passes current through coil L1.
When switching device Q1 is turned ON and OFF, the voltage from DC source 1 is intermittently applied to primary winding 5. Hence, a voltage is induced across secondary winding 7. This induced voltage is rectified and smoothed by circuit 9, whereby DC output voltage V.sub.o is produced. The output voltage V.sub.o is supplied to load 25 which is connected to output terminals 11,13.
A differential amplifier 15 compares DC output voltage V.sub.o with a preset voltage V.sub.r and produces an error signal according to differences therebetween, i.e. V.sub.r -V.sub.o. This error signal is fed back to a control circuit 23 while being isolated by a photocoupler 19. Control circuit 23 controls the ratio of conducting period of the switching device Q1 to the total period (of conducting and non-conducting periods of device Q1) so that the power supply will operate in such a manner as to substantially eliminate the error signal. That is, control circuit 23 performs a pulse-width modulation. Let T.sub.ON be the non-conducting period. If T.sub.ON /(T.sub.ON +T.sub.OFF) is increased, the amount of energy supplied to the secondary winding 7 via transformer 3 is increased and output voltage V.sub.o is increased. Conversely, if the above ratio is reduced, the output voltage V.sub.o is decreased.
An overvoltage or overcurrent may be produced at output terminals 11,13 for some reason. If so, load 25 may be damaged, or the power supply itself may be damaged. To prevent such desired problems, an excessive output detector 17 is provided to monitor the condition of the output signal. If an excessively large output V.sub.o is produced, detector 17 produces an output signal indicating that this excessively large output V.sub.o is being produced. A photocoupler 21 transmits this signal to control circuit 23 while concurrently isolating such signal. Control circuit 23 then no longer activates switching device Q1. Then, intermittent application of voltage V.sub.in to primary winding 5 is stopped. The voltage induced across secondary winding 7 disappears. The result is that DC output voltage V.sub.o becomes zero.
The switching power supply is normally installed in an environment where temperature varies greatly. Also, voltage Vin from source 1 supplied to the primary side of transformer 3 is changed greatly according to the condition of the power source. Under these conditions, it is not easy to stably transmit the error signal as an accurate analog signal from amplifier 15 to control circuit 23 because of the thermal characteristics of components of the power supply, such as semiconductor devices, and poor resistance of the analog network to noise.
Furthermore, it is not easy for the receiver section to extract different kinds of signals which are superimposed on each other and transmitted by the transmitter section. Accordingly, the circuit for transmitting the error signal and the circuit for transmitting the signal indicating the presence or absence of or excessively large output are provided separately. In the described conventional power supply, the error signal and the output signal from the excessive output detector are analog signals to be transmitted. The photocouplers 19 and 21 correspond to independent transfer circuits, and are used to perform electrical isolation between the circuits.
In recent years, electronic intelligence has enjoyed wider acceptance. With this increasing trend, the kinds of information transmitted between the secondary side of the power supply and the primary side thereof, are numerous in number and include the error signal, overcurrent output, overvoltage output, etc. Also, the amount of information being utilized is likely to increase. However, in conventional switching power supplies, each time a new kind of information is needed, an additional photocoupler is needed so that there is always the isolation between the information source and the control circuit using such information. This increases cost and amount of electrical power consumed.
Thus, conventional switching power supplies still leave much room for improvement.